From The National Geographic:
Hunting for food, ants roam haphazardly. But when they find it, they use celestial cues, perhaps from the sun, to head back to their nests more or less in a straight line—rather than retracing the tortuous journeys they’d made on their outbound searches. Instead, a new study suggests that ants have internal “pedometers,” or step counters, that help them gauge how far they have traveled. Food was placed about 33 feet (10 meters) from an ant nest. When ants found the food the researchers collected the insects before they had time to carry it back to the nest.
Twenty-five of the ants were then put gently on their backs. Scientists glued stilts made of pig bristles to the insects’ legs—a delicate procedure that had to be done quickly so the ants wouldn’t forget what they were doing and fail to return home. Another 25 ants had their legs surgically shortened by chopping off part of the bottom segment. For the ants on stilts, each step now covered more distance than they were used to. They overshot the nest, running an average of more than 50 percent farther than they should have. Those with shortened legs undershot by nearly as much.
More here.